Method for Exhaust-Gas Treatment for Diesel Engines or the Like, and Apparatus for Implementing This Method

ABSTRACT

In a method for exhaust-gas treatment for diesel engines, the exhaust gas is conducted through a regenerable particulate filter as well as an NOx catalytic converter, each disposed in exhaust branch. The exhaust gas is first conducted through the NOx catalytic converter of the carbamide SCR type situated close to the engine, and subsequently through the particulate filter of the CSF type, the energy necessary for regenerating the particulate filter being supplied to it from outside. Fuel is used as regenerating agent, which is taken from fuel tank provided for the operation of the diesel engine.

FIELD OF THE INVENTION

The present invention relates to a method for exhaust-gas treatment for diesel engines, etc., in which the exhaust gas is conducted through a regenerable particulate filter as well as an NOx catalytic converter, each disposed in the exhaust branch. The present invention also relates to an apparatus for implementing this method. Diesel engines include engines whose exhaust gases contain not insignificant quantities of soot particulates or similar residues from combustion.

BACKGROUND INFORMATION

It is conventional to reduce the content of soot particulates and nitrogen oxides in the exhaust gas using separate aggregates, to the extent that the exhaust gas complies with the exhaust-gas limiting values determined by law.

One technology proven for years for decreasing NOx is the carbamide SCR technology (SCR=selective catalytic reduction). In that case, carbamide or an aqueous carbamide solution is introduced via a metering system into the exhaust branch upstream of the NOx catalytic converter. Ammonia develops in two chemical steps in the NOx catalytic converter, and the ammonia reacts with the nitrogen oxides collected in the NOx catalytic converter to form nitrogen. The advantage of using carbamide is that its handling during transport and storage is completely problem-free, since carbamide is colorless, odorless, non-toxic and biologically safe.

Particulate filters of the CSF type (CSF=catalyzed soot filter) have proven to be reliable for filtering soot particulates out of the exhaust gas. They are particulate filters which have a catalytic coating in order to improve or permit the soot burn-off. Particulate filters of the CSF type are passive, that is, are continuously operating systems having high durability. A further advantage is their modest space requirement.

Therefore, NOx catalytic converters of the SCR type on one hand, and particulate filters of the CSF type on the other hand are particularly suitable for use in motor vehicles. Systems have already been discussed in the literature, in which a particulate filter of the CSF type and an NOx catalytic converter of the SCR type are disposed in the exhaust branch one behind the other relative to the direction of flow of the exhaust gas. Because of the relatively high exhaust-gas temperature, disposing the particulate filter close to the engine ensures a reliable soot burn-off. However, a problem results from disposing the SCR catalytic converter far from the engine. The carbamide SCR technology can first be used with sufficient stability starting from an exhaust-gas temperature of more than 200° C. Below this temperature, no fresh reducing agent can be injected into the exhaust branch, since a complete dissolution of the carbamide solution in the exhaust branch is not guaranteed at temperatures below 200° C. Namely, polymerization of the carbamide may occur, which leads to irreversible deposits in the exhaust branch and clogs it in the course of time.

This means that a significant removal of nitrogen oxide from the exhaust gas can only begin after the SCR catalytic converter has been heated up to a sufficient temperature after a certain operating time. Therefore, in an MVEG test cycle (MVEG=motor vehicle emissions group) in which the exhaust-gas temperature at the downstream SCR catalytic converter remains below 200° C. for a very long time, only a relatively low rate of nitrogen-oxide reduction can be expected.

SUMMARY

Example embodiments of the present invention provide a method, as well as an apparatus for implementing the method, in which the efficiency of the SCR catalytic converter may be improved in all operating ranges, while ensuring proper operation of the CSF particulate filter.

The method provides that the exhaust gas is first conducted through an NOx catalytic converter of the carbamide SCR type disposed close to the engine, and subsequently through a particulate filter of the CSF type, the energy necessary for regenerating the particulate filter being supplied to it from outside.

Because the SCR catalytic converter is located close to the engine, the work window of the catalytic converter is better utilized as a result of the higher exhaust-gas temperature. The conversion of the nitrogen oxides begins earlier, leading to an increase in the rate of nitrogen-oxide removal over the entire operating time (e.g., an MVEG test cycle). The carbamide necessary for the regeneration is injected into the exhaust branch upstream of the SCR catalytic converter.

To ensure proper operation of the CSF particulate filter despite being disposed at a distance from the engine, the energy necessary for its regeneration is supplied to it from outside. This energy may be supplied in different manners, namely, for instance, in the form of thermal energy with the aid of microwave- or electro-heating devices, or in the form of fuels which react with the oxygen surplus included in the exhaust gas. The fuel provided for the operation of the engine may be used as a regenerating agent for the CSF particulate filter, since this fuel is always available during operation and, for example, does not require a separate tank. In this context, however, the particulate filter cannot be regenerated using a mixture enrichment controlled by the engine management, since the exhaust-gas temperatures of more than 650° C. resulting therefrom—which are obtained, somewhat reduced, in the SCR catalytic converter as well—would clearly lie at its tolerance limits. Moreover, an increased hydrocarbon concentration caused by postinjection, upon passing over certain types of SCR catalysts, would lead to contamination of the catalyst, and therefore to losses in efficiency.

Therefore, the fuel is injected into the exhaust branch upstream of the particulate filter (HCl method).

Using the method according to example embodiments of the present invention, a good utilization of the work window of the SCR catalytic converter may therefore be achieved on one hand, as well as an engine-protecting particulate-filter regeneration on the other hand.

In example embodiments of the present invention, an apparatus includes an NOx catalytic converter of the carbamide SCR type disposed close to the engine, and a particulate filter of the CSF type downstream therefrom, and by a device assigned to the particulate filter for supplying the energy necessary for its regeneration. As already explained above, a device may be provided to inject fuel into the exhaust branch upstream of the particulate filter. For example, the fuel provided for the operation of the diesel engine may be used as fuel, so that the device for injecting fuel may be connected directly to the fuel tank of the diesel engine, and a separate fuel tank is not necessary.

When the apparatus described for exhaust-gas treatment is used in a motor vehicle, the particulate filter is disposed in the underbody area of the motor vehicle, where the heat output by it can be dissipated directly to the surroundings.

Example embodiments of the present invention are shown in the drawing and described in more detail below.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE schematically illustrates a motor vehicle having a combustion engine, in the exhaust branch of which, an SCR catalytic converter and a CSF particulate filter are situated one behind the other.

DETAILED DESCRIPTION

Motor vehicle 2 shown schematically in the FIGURE is powered by a diesel engine 4. As shown schematically, diesel engine 4 is equipped with an exhaust-gas turbocharger 6 which uses the exhaust gases of diesel engine 4 in a conventional manner to supercharge it.

The exhaust gases are carried away through an exhaust branch, denoted as a whole by numeral 8, into the surroundings. In this exhaust branch 8, directly behind diesel engine 4, an SCR catalytic converter 10 is situated, in which nitrogen oxide is removed from the exhaust gases in a first method step. SCR catalytic converter 10 works with carbamide or an aqueous carbamide solution as a reducing agent, which is taken from a carbamide tank 12 located, for example, in the rear end of the vehicle, and is injected into exhaust branch 8 upstream of SCR catalytic converter 10. Carbamide SCR catalytic converter 10 functions at an exhaust-gas temperature clearly within the heat-tolerance limits of the SCR catalytic converter in an effective range of the SCR work window.

Disposed in the exhaust branch downstream of the SCR catalytic converter is a regenerable particulate filter 14 which removes soot particulates from the exhaust gas. Particulate filter 14 is a particulate filter of the CSF type, which is coated with a catalytically active material. Particulate filter 14 is regenerated by fuel that is taken from fuel tank 16 of motor vehicle 2 and injected into exhaust branch 8 upstream of particulate filter 14. By a regulated metering of the fuel injection, it is possible to operate the particulate filter at a temperature optimal for the soot burn-off.

LIST OF REFERENCE NUMERALS

-   2 motor vehicle -   4 diesel engine -   6 exhaust-gas turbocharger -   8 exhaust branch -   10 SCR catalytic converter -   12 carbamide tank -   14 particulate filter -   16 fuel tank 

1-6. (canceled)
 7. A method for exhaust-gas treatment for a diesel engine, comprising: first conducting the exhaust gas through an NOx carbamide SCR catalytic converter disposed in an exhaust branch of the diesel engine and close to the diesel engine; subsequently conducting the exhaust gas through a CSF particulate filter disposed in the exhaust branch; and supplying energy from an outside to the particulate filter to regenerate the particulate filter.
 8. The method according to claim 7, further comprising injecting fuel into the exhaust branch upstream of the particulate filter.
 9. An apparatus for exhaust-gas treatment for a diesel engine, comprising: an NOx carbamide SCR catalytic converter disposed in an exhaust branch of the diesel engine and close to the diesel engine; a CSF particulate filter disposed in the exhaust branch and downstream of the catalytic converter; and a device configured to supply energy to the particulate filter to regenerate the particulate filter.
 10. The apparatus according to claim 9, further comprising a device configured to inject fuel into the exhaust branch upstream of the particulate filter.
 11. The apparatus according to claim 10, wherein the device configured to inject fuel is connected to a fuel tank of the diesel engine.
 12. The apparatus according to claim 9, wherein the particulate filter is disposed in an underbody area of a motor vehicle.
 13. The apparatus according to claim 9, wherein the apparatus is configured to perform the method according to claim
 7. 14. An apparatus for exhaust-gas treatment for a diesel engine, comprising: NOx carbamide SCR catalytic converting means disposed in an exhaust branch of the diesel engine and close to the diesel engine; CSF particulate filtering means disposed in the exhaust branch and downstream of the catalytic converting means; and means for supplying energy to the particulate filtering means to regenerate the particulate filtering means. 